Our visual systems undoubtedly process far more than we can actually remember. But what determines whether something is remembered or not? Here, I propose that the key to storage could be the duration of neuronal activation. Even when presented in an RSVP stream at 72 images per second, briefly presented stimuli can be fully processed and activate selective neurons in monkey IT (Keysers et al, 2001, J Cog Neurosci). However, this activation is usually very brief, lasting only a few tens of milliseconds. But for some stimuli, the activation could last longer for a range of possible reasons. For example, the image might match a target in a visual search task, allowing activity to be boosted by top-down feedback. Alternatively, the stimulus could be very surprising, or emotionally powerful. Whatever the reason, prolonged activation lasting for around 150 ms or so could be used to decide that the input pattern is worth storing in an episodic memory trace. I propose that measuring the duration of the cortical activation could be a key function of the hippocampus. Specifically, hippocampal neurons could effectively test whether the same cortical input pattern is present on two successive cycles of a hippocampal theta rhythm. If so, the hippocampal neurons should fire and allow the pattern to be stored. Such a temporal gating mechanism could explain the puzzling fact that hippocampal visual response latencies in humans are typically 280-300 ms – much longer that the neocortical neurons that provide their inputs and which probably become active at around 120-150 ms after stimulus onset. It would also explain the remarkably close match between hippocampal activation and conscious awareness (Quian-Quiroga et al, PNAS, 2008) because only consciously perceived stimulus would generate activation patterns that last long enough for storage.